Cell phones are being designed to include more bands and more modes than in the past. Under certain circumstances this can give rise to problems that degrade phone operation.
If a signal transmitted by a handset reaches a simultaneously operating receiver in the same handset at a strong enough level, it can saturate (“jam”) that receiver, and prevent that receiver from receiving information. Transmitters also produce noise, random low level signals occurring outside the intended band of transmission. If noise produced by a handset transmitter reaches a simultaneously operating receiver in the same handset at a strong enough level and at the frequency of operation of the receiver, it can degrade the signal-to-noise ratio of that receiver (“desense” that receiver), decreasing the ability of that receiver to accurately receive information.
Two common methods of keeping transmitted signals from interfering with received signals are Time Division Duplexing (TDD) and Frequency Division Duplexing (FDD).
In FDD, transmission occurs in different frequency bands than reception. If the system is also full duplex, i.e. transmission and reception occur simultaneously, then a frequency selective device called a duplexer is needed to prevent “jamming” and “desense”. Components of the duplexer include a transmit (Tx) filter and a receive (Rx) filter. The Tx filter is placed between the power amplifier (PA) and the antenna. This filter passes the transmitted signal with minimal attenuation, but provides high attenuation to any noise produced in the receive band, to prevent “desense”. The Rx filter is placed between the low noise amplifier (LNA) and the antenna. This filter passes the received signal with minimal attenuation, but provides high attenuation to any signal produced in the transmit band, to prevent “jamming”.
In TDD, transmission occurs at different times than reception. In this circumstance a transmit/receive (T/R) switch is commonly used to separate the receiver from the transmitter. If a system is both TDD and FDD, then if the transmitter is turned on and producing noise during the receive time slot, noise can “leak” across the T/R switch and “desense” the receiver. In this case a very high isolation switch is needed to avoid “desense”. Such a switch is challenging to manufacture and typically has more insertion loss than a switch that provides less isolation. Typically the need for a high isolation switch can be avoided by ensuring the transmitter is off and therefore not producing noise whenever the receiver is operating. Alternately, for a standard that is both TDD and FDD, a duplexer may be used as in the FDD case.
With the inclusion of multiple frequency bands in the same handset, a special circumstance can arise when operation in one of the frequency bands is full duplex FDD and a second required frequency band has the property that the transmit frequencies of the second band overlap the receive frequencies of the full duplex FDD band. In this case signals may pass through components necessary for transmission in the second band, and reach the receiver of the first band, resulting in either “jamming” or “desense” of the receiver of the first band.
An example of such a circumstance occurs in a so-called “Quad-band GSM, Tri-band UMTS” handset. The following bands are incorporated in this handset:
GSM 850 bandTx 824-849 MHzRx 869-894 MHzTDD, FDDGSM E-GSMTx 880-915 MHzRx 925-960 MHzTDD, FDDbandGSM DCSTx 1710-1785 MHzRx 1805-1880 MHzTDD, FDDbandGSM PCSTx 1850-1910 MHzRx 1930-1990 MHzTDD, FDDbandUMTS Band-1Tx 1920-1980 MHzRx 2110-2170 MHzFDD, fullduplexUMTS Band-2Tx 1850-1910 MHzRx 1930-1990 MHzFDD, fullduplexUMTS Band-5Tx 824-849 MHzRx 869-894 MHzFDD, fullduplexNote that UMTS Band-2 is FDD full duplex, and the UMTS Band-1 Tx frequencies overlaps the UMTS Band-2 receive frequencies.
A second significant circumstance is that UMTS systems require very linear front ends. Specifications for UMTS Band-1 called out in 3GPP TS 25.101 V3.13.0, section 7.6 require the ability to withstand a −15 dBm blocking signal (blocker) anywhere 0-2.025 GHz or 2.255-12.750 GHz while transmitting at a power level of +20 dBm. Analysis shows this translates into a Third Order Input Intercept Point (IIP3) requirement of greater than 62 dBm for the network in front of the UMTS Band-1 receiver. This is a far more stringent requirement that what is needed for operation to a GSM standard. Furthermore, this IIP3 reflects the cascade of all elements in front of the receiver, meaning that if multiple elements are required, such as duplexers and switches, each element must have sufficient linearity so that when combined, the resulting network exceeds the required 62 dBm IIP3. Thus the networks that combine the UMTS Band-1 receive path with the networks needed for other bands must be extremely linear.
In general the linearity of a switch decreases as more throws are added. Each additional throw has non-idealities that contribute incremental distortion and lower the intercept point of the switch. High isolation in switches is typically achieved by cascading throws; thus simultaneously achieving both high isolation and high linearity in the same switch is very challenging, with the situation worsening as more throws are added.
To date most handsets have not encountered the special circumstance described above. Common multi-band handsets include “Quad-band GSM” and more recently “Quad-band GSM plus UMTS Band-1”.
A “Quad-band GSM” handset includes the first four bands listed above. A typical architecture is shown in FIG. 1. The PAs for the 850 and E-GSM bands are combined into a single element, as are the PAs for the DCS and PCS bands. A single-pole six-throw (SP6T) switch is used to combine the bands to a single antenna, and also serves as the T/R switch for each band. A receive filter is used for each band to prevent jamming from out of band signals. Although the Tx of the PCS band overlaps with the Rx of the DCS band, and also the Tx of the E-GSM band overlaps with the Rx of the 850 band, since operation is TDD, the transmitters and receivers are not operating simultaneously so “desense” and “jamming” are avoided. The T/R switch has only moderate isolation requirements. Since no UMTS signals occur in this system, there is no high linearity requirement on the T/R switch.
A “Quad-band GSM plus UMTS Band-1” handset includes the first 5 bands listed above. A typical architecture is shown in FIG. 2. A SP2T switch is used to combine the UMTS Band-1 circuitry with a Quad-band GSM network such as is described above. The UMTS Band-1 circuitry includes a duplexer, a separate PA, and receive and transmit filters. Although UMTS Band-1 is FDD full duplex and there is overlap between PCS and UMTS Band-1, it is the Tx of the UMTS Band-1 band that overlaps with the Rx of the PCS band. As the UMTS Band-1 transmitter and the PCS receiver are not operated at the same time (so called “compressed mode”), “desense” and “jamming” are avoided. The SP2T switch needs a high intercept point in order to handle the UMTS Band-1 signals, but this requirement is mitigated by minimizing the number of throws needed. As the PA for UMTS Band-1 is separate from the PA for PCS, it can be turned off during PCS operation. This removes the noise source, so eliminates the need for high isolation in the SP2T switch.